1

Trace element and Pb isotope fingerprinting of atmospheric pollution sources: 1

A case study from the east coast of Ireland 2

3

Carolina Rosca1,3 *, Emma L Tomlinson1, Walter Geibert2, Cora A McKenna1, 4

Michael G Babechuk1, 4 and Balz S Kamber1 5

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1Department of Geology, Trinity College Dublin, Ireland 7

2Alfred-Wegner-Institut Helmholtz-Zentrum für Polar- und Meeresforschung, Bremerhaven, 8

Germany 9

3present address: Department of Geosciences, University of Tübingen, Germany 10

4present address: Department of Earth Sciences, Faculty of Science, Memorial University of 11

Newfoundland, Canada 12

13

*corresponding author: roscaca@tcd.ie 14

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Abstract 15

Unravelling inputs of multiple air pollution sources and reconstructing their historic contribution 16

can be a difficult task. Here, trace metal concentrations and Pb isotopes were measured in a 17

radionuclide (210Pb-241Am) dated peat core from the Liffey Head bog (LHB) in eastern Ireland 18

in order to reconstruct how different sources contributed to the atmospheric pollution over the 19

past century. Highest enrichments in the heavy metals Pb, Cu, Ag, Sn, and Sb, together with 20

a Pb isotope composition (206Pb/204Pb: 18.351±0.013; 206Pb/207Pb: 1.174±0.012) close to that 21

of the Wicklow mineralisation demonstrates significant aerial influx of heavy metals from local 22

mining and smelting activities during the 19th century until ca. 1940’s. A dramatic compositional 23

shift defined by elevated Co, Cr, Ni, Mo, Zn, and V enrichments and a sharp transition towards 24

unradiogenic 206Pb values (206Pb/204Pb: 18.271±0.013 - 17.678±0.006; 206Pb/207Pb: 25

1.170±0.012 - 1.135±0.007) is documented from the 1940’s until ca. 2000. These are attributed 26

to the atmospheric impact of fossil fuels and especially leaded petrol, modelled to have con-27

tributed between 6 and 78% to the total Pb pollution at this site. The subsequent turn to a more 28

radiogenic Pb isotope signature since 2000 in Ireland is clearly documented in the investigated 29

archive (206Pb/204Pb: 17.930±0.006; 206Pb/207Pb: 1.148±0.007) and reflects the abolishment of 30

leaded petrol. However, there remains a persisting and even increasing pollution in Ni, Mo, 31

Cu, and especially Zn, collectively originating from countrywide use of fossil fuels (peat, coal, 32

heating oil, and unleaded vehicle fuels) for domestic and industrial purposes. This illustrates 33

the continued anthropogenic influence on important natural archives such as bogs in Ireland 34

despite the phase-out of leaded petrol. 35

36

Key words: anthropogenic pollution, heavy metals, Pb isotopes, ombrotrophic peatland, Ire-37

land 38

39

3

1. Introduction 40

Anthropogenic activities are responsible for significant perturbations of the natural heavy metal 41

cycles at the Earth’s surface since at least 3000 years (Fábregas Valcarce et al., 2003). Om-42

brotrophic peatlands are considered excellent archives of historic atmospheric composition 43

due to two key characteristics: i) the hydrological isolation from ground water and surface run-44

off ensures that the growing peat is exclusively fed by atmospheric inputs (i.e., dust, rain, snow, 45

fog); and, ii) the inherently high abundance of complex-forming organic acids, together with a 46

pH ranging between 4 and 5 enables the preservation of metal-bearing aerosols at the depth 47

of their deposition (e.g., Shotyk and Le Roux, 2005; Zaccone et al., 2009). There is an increas-48

ing body of publications on historic atmospheric trace metal depositions, especially of Pb, Zn, 49

Cu, Hg, As, Cd inferred from study of bogs from around the world, including Canada (e.g., 50

Boyle, 1977; Pratte et al., 2013; Shotyk, 1992), Europe (e.g., De Vleeschouwer et al., 2009; 51

Martínez Cortizas et al., 2012; Shotyk et al., 2002), Australia (e.g., Marx et al., 2010; Stromsoe 52

et al., 2015), China (e.g., Ferrat et al., 2012), and South America (e.g., De Vleeschouwer et 53

al., 2014). These studies report temporal variations in natural dust depositions, while also doc-54

umenting significant changes in the atmospheric metal pollution load throughout the periods 55

of modern industrialisation into recent times (i.e., post Pb-gasoline). 56

Radiogenic Pb isotope analyses are a robust tool to distinguish between natural and 57

anthropogenic Pb inputs due to conservative behaviour in bogs, including very low mobility 58

and insignificant post-depositional isotope fractionation (Shotyk et al., 2005 and references 59

therein). The isotopic signature of Pb, which shows a broad compositional variability within the 60

lithogenic units found at the Earth’s surface is determined by the geological age and respective 61

U-Th-Pb concentrations of the parent rock. For example, Precambrian Pb ores from the 62

Brocken Hill deposit at Mt. Isa, Australia display an unradiogenic 206Pb/207Pb of 1.04 (Town-63

send and Snape, 2002), whereas the Missisippi type Pb deposits in the USA carry a radiogenic 64

206Pb/207Pb signature ranging between 1.28 and 1.33 (Doe, 1970). Products manufactured us-65

ing these ores will inherit their respective isotope signature. The pre-polluted 206Pb/207Pb values 66

of Greenlandic ice (Rosman et al., 1997), bogs from Spain (Kylander et al., 2005), Switzerland 67

4

(Shotyk et al., 2001) as well as Sweden (Klaminder et al., 2003) range between 1.19 and 1.25. 68

The temporal and geographic evolution of the Pb pollution fingerprint has helped to discrimi-69

nate between the impact of mining-smelting and leaded gasoline around the northern hemi-70

sphere (e.g., Bindler et al., 2004, Bränvall et al., 1997; Cloy et al., 2008; Kylander et al., 2005, 71

2009; Le Roux et al., 2004, 2005; Martinez-Cortizas et al., 2012; Shotyk et al., 2002). While 72

often deployed separately, the information obtained from combining trace elements and (Pb) 73

isotopic compositions is more powerful in differentiating between complex multiple sources. 74

To date, there is not a single study reporting Pb isotope chronologies in Irish peatlands, and 75

only two studies which investigated atmospheric depositions of Pb, Cd, and Hg: Kippure bog, 76

Clara bog, Bellacorick bog (Schell et al., 1997), Knockroe bog, and Letterfrack bog (Coggins 77

et al., 2006), which, compared to other European areas is rather scarce. There is thus a strong 78

necessity to geochemically explore the Irish archives in more detail, especially in view of the 79

rich metallurgical history of the island and Ireland’s potential subjection to trans-Atlantic metal 80

transport. 81

In this study, we investigate the elemental pollution history in a bog from the Wicklow 82

Mountains in eastern Ireland with a particular focus on the last century. The Wicklow uplands 83

were Ireland’s most important lead mining centre from 1824 AD until the first half of the 20th 84

century (Schwartz and Critchley, 1996). The area documents extensive lead-silver-zinc and 85

copper (Avoca) extractions, with ore dressing taking place on site and smelting conducted at 86

Glenmalure (parallel valley), as well as in a coal operated smelter ~35 km NE from the mining 87

locality at Ballycorus (Rynne, 2015). Subeconomic conditions coupled with poor infrastructure 88

and a drop in Pb prices triggered by the South American market surplus resulted in closure of 89

all mining activities in the Wicklow uplands in the 1940’s-1950’s. The Liffey Head peat (LHB) 90

archive was selected for investigations due to its proximity to the metallurgical sites and its 91

location at the east coast of Ireland, allowing also an investigation of trans-regional metal 92

transport on easterly air streams. Further to the metals of interest (Pb-Zn-Ag-Cu), this study 93

also investigates the enrichment histories of other heavy metals such as Sn, Sb, Co, Cr, Ni, 94

Mo, and V deposited onto LHB. This is particularly important because most of these metals 95

occur as impurities within Irish Cu and Pb-Ag-Zn mineralisation, while also associated with 96

5

emissions from combustion of fossil fuels, steel and iron manufacture, and refuse incineration 97

(Nriagu and Pacyna, 1988). By combining the information gained from trace element and Pb 98

isotope compositions, our aim is to deconvolute multiple input sources that have influenced 99

the atmospheric metal composition over the last century at this site. 100

101

2. Background and methods 102

2.1. Study area and sample collection 103

In the Republic of Ireland, where ca. 1/6th (~1,200,000 hectares) of landmass is covered by 104

peat, three basic peat formations are recognised: 1) raised bogs of the Central Plain, 2) blanket 105

bogs dominating the western seaboard and the upland areas, and 3) fen peats (Hammond, 106

1978). Most of the Irish blanket and montane bogs are around 6000-8000 years old with aver-107

age accumulation rates of 0.1-1.2 cm year-1 (Hammond, 1978). The site investigated here, 108

LHB in Co. Wicklow (N 53°09’32”, W 6°17’33”) is an ombrotrophic, montane-type blanket peat-109

land situated 25 km southwest of Dublin city and approximately 15 km from the east coast of 110

Ireland (Fig. 1). Liffey Head occurs as a relatively flat bog with a series of pool complexes and 111

lies between 490 and 520 m above sea level. The annual precipitation in this part of the Wick-112

low Mountains is high, with means between 1600 and 2400 mm a-1 (source: Meteorological 113

Survey). These conditions sustain a near-continuous deposition of atmospheric particulates 114

onto the bog’s surface. The upland area experiences aerosol transport from both westerly At-115

lantic winds, potentially carrying pollutants from mines in the midlands, and injections of east-116

erly air masses which direct pollutants from the UK and beyond (e.g., Bowman and McGet-117

tigan, 1994; Feeley et al., 2013). The bog is underlain by the Leinster batholith, a Caledonian 118

age granitic intrusion with a strong I-type, calc-alkaline affinity (e.g., Oliver et al., 2002). 119

A vertical monolith of 115 m in length (10x10x115 cm) was extracted from the LHB in 120

March 2015 from an area with living Sphagnum moss growth on top using a stainless steel 121

Wardenaar corer. The core was sampled close to the centre of the bog at its highest elevation. 122

On site, the core was wrapped in cling film and aluminium foil, placed in a wooden box, brought 123

directly to the laboratory and stored in a dark room at ca. +2ºC. Using a thin ceramic knife, the 124

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core was cut into 115 one cm slices, each apportioned for individual analyses as described in 125

detail below. 126

127

2.2. Radiometric data and analysis 128

The radioisotopes 210Pb, 137Cs, and 241Am were analysed via gamma-spectrometry on a sub-129

set of samples to derive a chronology for the top section of the core. All analyses were per-130

formed on a planar HPGe gamma detector (Canberra) at the Alfred Wegener Institute for Polar 131

and Marine Research in Bremerhaven, Germany. Peat samples were weighed and sealed with 132

hot glue in gas-tight petri dishes to prevent loss of ingrowing 222Rn. Subsequently, samples 133

were stored for >3 weeks to allow the relevant daughters of 226Ra to grow into secular equilib-134

rium. Lead-210 was measured at 46 keV, 241Am at 59 keV, and 137Cs at 661 keV. The presence 135

of 226Ra was monitored at lines 186 keV, 295 keV, 351 keV, and 609 keV. Detectable 226Ra 136

was found in fewer than 50% of the samples, but its activity was less than 1% of the total 210Pb. 137

Consequently, no correction for supported 210Pb was performed to avoid introducing unneces-138

sary inconsistencies through the profile. Lead-210 is not reported as excess 210Pb (210Pbex), 139

while noting that nearly all 210Pb will be unsupported. Samples were counted until 1000 net 140

counts of 210Pb were reached, or for maximum of 6*105 seconds if 1000 counts were not 141

reached within this time period. Variable sample masses were expected to affect the detector 142

efficiencies via self-absorption, especially at the low energy range. This was addressed by 143

determining mass-dependent efficiencies using IAEA-385 Irish Sea reference material for 144

210Pb and 241Am. The uncertainty on these efficiencies was assumed to be 10% (1sd). For 145

137Cs, an uncertainty of 5% (1sd) was expected, taking into account the inevitably variable 146

geometry of the unprocessed peat samples. Counting errors were typically small compared to 147

the uncertainty in the efficiencies. Therefore, we applied an error calculation based on the error 148

propagation from detector efficiencies and counting statistics, without including the nominal 149

uncertainty in the background determination. 150

151

2.3. Bulk peat properties 152

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Moisture contents were determined by drying a 2 cm3 subsample (2x1x1 each cm piece of 153

peat) at 105ºC for 24 hours. The organic matter contents were defined via the loss on ignition 154

(LOI) by combusting the dried aliquot at 550ºC for 6 hours in inert quartz glass crucibles. Bulk 155

density (g cm-3) was analysed at 5 cm resolution for the deepest part (40-100 cm) and at 3 cm 156

resolution for the section between 0 and 40 cm following the method of Dean (1974). 157

158

2.4. Trace element and Pb isotope analyses 159

Trace element concentrations in peat samples were acquired at Trinity College Dublin, Ireland 160

via solution quadrupole ICP-MS (SQ-ICP-MS) using a Thermo Scientific iCAP-Qc, and follow-161

ing the method described in Marx et al. (2010). A dedicated ash fraction was prepared for this 162

purpose by combusting a peat aliquot for 6 hours at 450ºC. Between 10 and 50 mg of resulting 163

ash was transferred into 15 ml Teflon beakers into which 0.8 ml of a triple sub-boiling distilled 164

conc. HF and 0.2 ml conc. HNO3 (4:1) was added. The capped beakers were placed into a 165

digestion block at 120ºC for 72 hrs and agitated at least once every 24 hrs in a HEPA-filtered 166

fume cupboard. After cool-down, drops were carefully collected before beakers were opened, 167

and the solution was evaporated at 110ºC. The residues were then dissolved with 1 ml of 6 M 168

HCl to reduce any remaining organic components, and fluorides were converted by attacking 169

with 2x0.5 ml conc. HNO3 with evaporation to dryness between each step. Finally, the con-170

verted residue was dissolved in 3 ml of 2.5 M HNO3 and subsequently topped up with additional 171

Milli-Q water to produce 30 ml of 0.3 M HNO3 solutions. Two blanks were prepared along with 172

the samples throughout all ashing and digestion steps to account for potential contamination. 173

174

The ICP-MS analyses followed the procedure of Eggins et al. (1997) with modifications de-175

scribed by Babechuk et al. (2010) and Marx et al. (2010). For analyses, 2 ml solutions contain-176

ing a small aliquot of each stock solution was diluted with 2% HNO3 and spiked with an internal 177

standard containing a mixture of 6Li, Rh, Re, Bi and 235U, which is used for instrumental drift 178

correction, covering the full mass range of ionisation potentials and analysed elements. In 179

some cases, the sample contained so much Bi that this affected the signal from the internal 180

standard, which was then excluded for drift correction (interpolating instead between Re and 181

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U). Following Eggins et al. (1997), an additional external drift monitor was employed. Oxide/hy-182

droxide and dimer interferences were corrected according to Ulrich et al. (2010). Multiple di-183

gestions of the USGS W-2a reference material were used for calibration, and additional USGS 184

reference materials BCR-2 and BHVO-2 were measured throughout the duration of the study 185

as the quality control standards. One relative standard deviations (rsd) of the mean of multiple 186

measurements of the respective standards were typically <2% for BHVO-2 and BCR-2 and the 187

concentrations were comparable to the GeoReM preferred values (Jochum et al. 2016) 188

demonstrating very good accuracy (low bias) of this dataset (Appendix A.2). The elements with 189

poorer reproducibility (>2% rsd) and/or higher bias relative to preferred values are those at 190

very low ng g-1 concentrations or those known to be heterogeneous in the USGS materials 191

(e.g. Kamber and Gladu, 2009; Weis et al., 2006). The concentrations determined for this study 192

also agree well with longer term determinations on the reference materials from similar tech-193

niques that applied the same calibration values (e.g., Babechuk et al., 2015; Kamber, 2009). 194

Once element concentrations were acquired, the previously determined ash content 195

obtained at 450°C for was used to back-calculate the element concentration in dry (pre-ashed) 196

peat. These concentration values (in ng g-1) are reported in Appendix A.1. Dry peat concentra-197

tions were used for the calculation of enrichment factors (EF) and excess pollution. 198

199

Lead isotope ratios of all peat samples analysed for trace element composition (n=41) were 200

measured on the same quadrupole ICP-MS at Trinity College Dublin. Because trace element 201

analyses consumed only a small fraction of the digest (<1%), the remaining stock solution was 202

used for Pb purification on anion exchange resin (AGX-1x, 200-400 mesh) applying the HBr-203

HCl method described in Kamber and Gladu (2009). Total Pb yield was between 85-90% for 204

all samples and the purity of the Pb solution was verified with a fast mass scan prior to isotope 205

ratio analysis. Procedural blanks were negligible with amounts less than 0.1% of the total Pb. 206

The quadrupole ICP-MS Pb analysis technique used followed Ulrich et al. (2010). Accuracy 207

and precision of the Pb isotope ratios were determined from multiple analyses of SRM-NBS 208

982 standard solution as shown in Appendix A.3. Bias was 95 ppm for 206Pb/204Pb, 44 ppm 209

208Pb/206Pb, and 25 ppm for 207Pb/206Pb, relative to the values of Baker et al. (2004). Lead 210

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isotope ratios of the rock-reference materials BHVO-2 (n=2) and BCR-2 (n=2), presented in 211

Table 2 are close to or within the values reported by Woodhead and Hergt (2000) and Baker 212

et al. (2004), demonstrating the good accuracy of this dataset, considering some heterogeneity 213

in Pb isotope composition in these reference materials as a result of their contamination during 214

powder preparation, as discussed elsewhere (e.g., Weis et al., 2006). 215

We note that in environmental studies, the most commonly used Pb isotope data presen-216

tation is the use of the following ratios: 206Pb/207Pb or 206Pb/204Pb. While the former is justified 217

by a better analytical precision, a normalisation to 204Pb will result in the largest possible vari-218

ability between reservoirs. Furthermore, as previously pointed by Ellam (2004), an omission to 219

include 204Pb in the data interpretation might result in a limitation of source characterisation. 220

In this study, we performed the source apportionment with the conventional Pb diagram 221

(207Pb/204Pb vs. 206Pb/204Pb) which offers the best dispersion. For ease of reference and com-222

parison to previous studies, the calculated 206Pb/207Pb values are also included. 223

224

3. Results 225

3.1. Radionuclide chronology reconstruction 226

The detection limit of 210Pb is located at 23-24 cm from the surface, indicating that almost the 227

entire 210Pb inventory is captured within this peat segment. We find a 210Pb inventory of 4830 228

Bq/m2. The 210Pb profile (Fig. 2a.) is relatively complex and does not show a monotonous de-229

crease with depth thus impeding the use of a constant initial concentration (CIC) approach. 230

The more suitable 210Pbxs-chronology is therefore a constant-rate-of-supply (CRS)-model (Ap-231

pleby and Oldfield 1978). The CRS-model can take variations in mass accumulation rates into 232

account, while still relying on a constant supply rate of 210Pb, the absence of initial penetration 233

(IP) of 210Pb, and a complete assessment of the 210Pbxs inventory. Such a model for our core 234

is shown in Figure 2e. The possible initial penetration of 210Pb in peat has been described 235

elsewhere and may in principle be modelled, assuming a constant initial penetration and a 236

constant rate of supply of 210Pb (IP-CRS model, Olid et al. 2016). However, this model may 237

still show discrepancies to the record of artificial radionuclides (Olid et al. 2016), in particular 238

241Am, which is a robust age marker, while not providing a continuous age record. We therefore 239

10

used constraints based on 241Am together with the distribution of the 210Pb inventory in order 240

to derive the best possible age information. 241

Assuming that 210Pb is not mobile, the complete inventory reflecting 4-5 half-lives is found 242

in the top 24 cm. This implies a limit of 88-110 years at 24 cm, close to the CRS dates, trans-243

lating into an average sedimentation rate of 0.22-0.27 cm yr-1. Slower accumulation would be 244

expected if 210Pb was somewhat mobile, but still within the calculated upper limit. The sudden 245

increase in 210Pb activity at 8.5 cm cannot be assigned to a change in supply rate, but likely 246

represents a sudden collapse in the growth rate. 247

Americium-241 (Fig. 2b.) was found to be the least mobile among the investigated radi-248

onuclides, in agreement with findings of other studies (Gallagher et al., 2016). It was present 249

at much lower activities and it was detectable only in few samples. The broad 241Am peak at 250

10-11 cm likely reflects the atmospheric release from nuclear tests in 1963-1964 (e.g., Gal-251

lagher et al., 2005). This would translate to a record of 50 years with an average accumulation 252

rate of 0.21 cm yr-1 for the uppermost 11 cm. However, this is a minimum value as 241Am is 253

also found at about half this activity in the sample below (at 12-13 cm), leaving the possibility 254

that the actual 241Am peak can be extended down to 12 cm depth. A secondary, smaller 241Am 255

signal is found at 16.5 cm. This is in slight disagreement with 210Pbxs CRS-model as 1963 would 256

be expected below 17 cm. 257

Two additional samples investigated at depths of 70-71 cm and 94-95 cm from the sur-258

face yielded no 210Pb and 241Am activity, but a small signal of 137Cs (4.4 Bq kg-1, Fig. 2c.). This 259

observation suggests significant down core Cs mobility at our site, as also suggested from the 260

plots in Appendix B.1, meaning that 137Cs is of little value in providing precise chronological 261

information here. Caesium mobility has been demonstrated in several studies of organic, clay-262

rich, low pH atmospheric and terrestrial archives, limiting its application as a chronologic tool 263

(e.g., Appleby et al., 1991; Kudelsky et al., 1996). Consequently, 137Cs activity was excluded 264

from the chronology reconstruction of the LHB peat profile. 265

Combined, the activity profiles obtained from 210Pb (S<0.27 cm yr-1) and 241Am (S >0.21 266

cm yr-1, with a depth trend) yield an average accumulation of 0.24 ±0.03 cm yr-1 for the upper-267

11

most 24 cm of the LHB peat core. This result is in agreement with the average values calcu-268

lated for Irish bogs (Hammond, 1978). The 210Pb-241Am age-depth relationship for the upper-269

most 24 cm was derived based on the above information, and the results expressed as AD 270

ages are presented along with element enrichment profiles in Figure 3 and 4. 271

272

3.2. Bulk peat properties 273

The organic matter (Fig. 2d.) and moisture contents are relatively constant throughout the core 274

with values ranging between 96-99% and 88-96% (by wt.), respectively. Density (Fig. 2c.) 275

ranges between 0.023-0.034 g cm-3, with the highest values detected in the segment between 276

10 and 40 cm from the surface. Subtle fluctuations in the moisture and density contents (es-277

pecially in the uppermost 25 cm) indicate minor down-core compaction and decomposition, 278

favouring element retention at deposition depth. High organic matter contents (or low ash con-279

tents) evidence low input of atmospheric dust, validating the ombrotrophic nature on the peat-280

land. The fluctuations are strongly anti-correlated with lithophile elements Ti (r2= 0.69), Sr (r2= 281

0.74), Sc (r2= 0.75), Ta (r2= 0.95), and Zr (r2= 0.8), supporting that mineral dust is the dominant 282

component of the residual incombustible ash (e.g., Shotyk et al., 2002). 283

284

3.3. Trace element patterns along the extracted LHB peat monolith 285

3.3.1. Geochemical data treatment 286

A principal component analysis (PCA) has been suggested as a strategy to minimize the 287

amount of factors governing enrichment in multi-element datasets such as the one presented 288

here (e.g., Küttner et al., 2014; Martínez Cortizas et al., 2013; Muller et al., 2008). The main 289

advantage of a factor analysis is that it allows the elements to be source-grouped according to 290

their distribution patterns and loadings. We applied such PCA model for our dataset using the 291

statistical software package PAST3 (Hammer et al., 2001). The results and interpretations are 292

shown in Appendix C. In brief, three main components are able to explain 99.75% of the total 293

variance. Factor 1 (PC1) accounts for the largest proportion of the variation (92.7%) with the 294

other two factors, PC 2 and PC 3, accounting to 4.4% and 2.7% respectively. PC1 includes 295

the elements Pb, In, Sn, Sb, Ga, Cd, Cu, Zn, Ag, Ti, with Pb showing the highest association 296

12

of 97%. PC2 groups the elements Ti, Zr, Ta, Ba, Li, As, Tl, Sr, and Rare Earth elements (REE) 297

together, and PC3: Zn, W, Mo, Ni, Ti, Sr, Cr, and Fe. While 77% of Zn is shared with PC3, 298

almost 15% of it is also shared with PC1, pointing to multiple sources controlling Zn enrichment 299

in the LHB peat. Based on their element assemblages, the three element groups can be care-300

fully attributed to dust contributions from the historical Pb-Zn mining and smelting, natural ge-301

ogenic dust input, and emissions from fossil fuel combustion, respectively (Appendix C). The 302

latter is yet to be characterised in more detail. We find that for the LHB dataset, the PCA 303

approach offers a first insight into the major pollutant groups contributing to the element en-304

richment during the past century. However, in light of complex element apportioning to the 305

respective components from co-occurring pollutants, the information obtained by the PCA is 306

combined with other geochemical tools to better fingerprint sources contributing to metal en-307

richment in LHB. 308

309

3.3.2. Enrichment factors (EF) 310

When investigating anthropogenic pollution, upper crustal values (UCC) are sometimes used 311

as a reference to extract the natural dust contributions. In doing so, metal enrichments are 312

discussed relative to geogenic element backgrounds with a poorly soluble lithophile element, 313

such as Ti, Sc, Sr, Zr, Al, Si, Y, Ta, or REE as a normalizer (e.g., Espi et al., 1997; Kempter et 314

al., 1997; Marx et al., 2010; Shotyk, 1996). The use of the estimated UCC values as a measure 315

for geogenic input has been, however, intensively debated in several studies (e.g., Martínez 316

Cortizas et al., 2002; Reimann and De Caritat, 2000). The main criticism is that the calculated 317

UCC values represent an average composition that is not necessarily representative of the 318

local lithological background. This can ultimately result in an under- or overestimation of the 319

natural geogenic budget transported into an archive. To overcome this issue, the interpretation 320

of the element EF’s relative to a local “baseline”, representative for the area of interest has 321

been proposed (e.g., Martínez Cortizas et al., 2002). This strategy is preferred for this study 322

due to the versatile Irish geology, and trace element patterns are presented as enrichment 323

factors (EF, Table 1) with respect to the average of the least polluted and compositionally most 324

13

consistent part of the core (62 and 100 cm from the surface). This “baseline” is used to calcu-325

late element EF’s at shallower depths, as described in [1]. 326

[1] EF = [(metal/Ta)sample /(metal/Ta)baseline] 327

328

The EF were calculated using the element concentrations in dried peat (at 105°C). Although Ti 329

shows the highest loading (80%) within PC2 (geogenic dust), its partial association with PC1 330

(16% mining-smelting of local ores) and PC3 (Appendix C) restricts its application as natural 331

dust indicator. We observe that, within this core, the best correlation with the residual ash (i.e., 332

geogenic dust content) is built with Ta (r2= 0.95, Appendix B.2). This is a strong argument that 333

Ta is the best candidate to extract dust depositions in our core. Tantalum has been shown to 334

be an excellent proxy for geogenic input (e.g., Babechuk et al., 2015; Marx et al., 2010), but 335

its successful analysis requires an acid-digest with HF of high purity, as employed here. 336

The REE patterns show a typical UCC-like signature consistent with the nearby granitic 337

bedrock with enrichments in the light REE over heavier REE (Appendix B.3). The REE patterns 338

remain subparallel throughout the entire core, indicative of a homogeneous geogenic origin of 339

the atmospheric dust. 340

341

3.4. Excursions of element enrichment 342

All trace element patterns show well-defined excursions of enrichments along the extracted 343

LHB peat profile (Table 1 and Appendix A.1 for raw concentrations). In Figure 3 we show a 344

representative selection of the results. Lead, Cu, and Zn depositions increase jointly from 62 345

cm depth (Fig. 3 and 4) and peak at different stages during the 20th century (Pb and Cu) until 346

recently (Zn). Enhanced deposition is observed in a number of other metals which asynchro-347

nously reach their maxima between 1940 and 1960 (Ag and Sb), 1960 and 1970 (Ni), 1950’s 348

and recently (Mo). Rare earth elements (REE) are highest between 10 and 20 cm, i.e., first 349

half of the 20th century, to then decrease again significantly in recently growing Sphagnum 350

moss (Appendix B.3). A rapid decline in their enrichments is also observed for a number of 351

other elements (e.g., Ag, Pb, Sn, Sb, In, Cd, U; not shown) between 1970 - present, whereas 352

14

enrichments of Ni and Cu do not quite return to pre-anthropogenic levels and show diminished 353

present-day atmospheric fallout (Fig. 3 and 5). Enrichment factors of Mo and Zn increase more 354

rapidly from ca 1940 and reach highest values of 5 and 39, respectively in the sub-surface 355

level of LHB. 356

357

3.5. Lead isotope compositions 358

Lead isotope ratios are presented together with EF of Cu, Zn, and Pb in Figure 4. Lead isotope 359

composition of the peat is homogenous from the base of the core until ca. 18 cm (206Pb/204Pb: 360

18.271-18.429, 206Pb/207Pb: 1.171-1.179). A dramatic change towards unradiogenic Pb signa-361

ture, coincident with major EF peaks for Cu and Pb is seen between 18 cm and 9 cm, where 362

a small plateau is reached (206Pb/204Pb: 17.712-18.123, 206Pb/207Pb: 1.138-1.161). Most un-363

radiogenic values (206Pb/204Pb: 17.657-17.678, 206Pb/207Pb: 1.135-1.136) are recorded at the 364

depth interval between 8 and 4 cm from the surface. This period is followed by a reversal 365

towards more radiogenic Pb isotope signature that extends up to the top of the bog. In detail, 366

however, the modern isotope signature (206Pb/204Pb: 17.746-17.929, 206Pb/207Pb: 1.137-1.148) 367

does not quite return to the values below 18 cm. 368

369

4. Discussion 370

4.1. Temporal patterns in metal enrichments and implications for element mobility 371

Conservative behaviour of at least some elements within bogs is a prerequisite for their inter-372

pretation of atmospheric pollution histories. Experimental studies have demonstrated that 373

strong complexation and fixation of metals onto the organic matter occurs in acidic environ-374

ments such as bogs (e.g., Pokrovsky et al., 2005a). The retention of the elements at the depth 375

of deposition is aided by carbon oversaturation within biomass and pore water (Smith et al., 376

2004). Krosshavn et al. (1993) have shown that metals such as Cu, Zn, Cd, and Pb can some-377

times display variable binding capacities, depending on the vegetational background. Yet, the 378

most favourable condition for the successful retention of metals onto the organic matter has 379

been found to be at a pH of 4. The surface of LHB displays pH conditions ranging between 4 380

and 4.5. 381

15

Little is known about the behaviour of transition metals and especially V, Cr, Ni, Co, Mo 382

in peat archives. Therefore, interpreting their behaviour is often aided by the comparison to a 383

relatively immobile metal, such as Pb. For example, by investigating their patterns in an om-384

brotrophic peatland from the Swiss Alps, Krachler et al. (2003) and Krachler and Shotyk (2004) 385

have found that V, Cr, Ni, Co, Mo are effectively immobile within the peat column. The spatio-386

temporal enrichments of these elements were interpreted to render emissions from various 387

anthropogenic sources such as steel production and combustion of fossil fuels. 388

In LHB, Cd, Ag, Sn, Sb, In, Co, Mo, Cr, and V are well-correlated with Pb (R2=0.71 to 389

0.88) which is commonly accepted to be highly immobile in peat (Martínez Cortizas et al., 2012; 390

Shotyk, 1996). Stronger shared variations are observed between Pb-Ag-Sn-Sb-In (e.g., Ag vs. 391

Sb: R2=0.93), and Co-Mo-Cr-V-Zn (e.g., Co vs. V: R2=0.91) dividing this wider group of ele-392

ments into two suites. As suggested from the PCA, these elements correspond to the PC1 and 393

PC3 groups. Further, the temporal deposition patterns of the Zn-Co-Mo-Cr-V (PC3) group dif-394

fer from those of Pb-Ag-Sn-Sb-In (PC1) by displaying a continuing upward trajectory of their 395

enrichment when PC1 shows a decrease in its pollution load (Fig. 3 and Appendix C). The 396

onset of Zn enrichment is coincident with that of Pb pointing towards a simultaneous pollution 397

initiation rather than a down-core mobilisation of Zn. The diverging Pb and Zn enrichment pat-398

terns towards the top of the core, i.e., over the last 50 years (R2=0.4, Fig. 4) reflect either i) the 399

decreasing industrial use of the former (PC1) and increased utility of the latter (PC3), or ii) a 400

different source/process responsible for the enrichment of metals in subsurface-layers of the 401

bog. Minor Zn bioaccumulation in the living part of the bog cannot fully be excluded. However, 402

its relevance to the total Zn budget and source apportioning is rather insignificant as shown 403

from its >95% assignment to PC 1, 2, and 3. In the following chapter, we discuss in detail the 404

complex behaviour of element enrichments in LHB in light of the known records of Irish indus-405

trial development. We combine the information obtained by PCA, EF’s, and Pb isotopic com-406

position, and discuss the pollution patterns from the base of the core towards the surface, as 407

chronologically subdivided in Figure 4. 408

409

4.2. The baseline (100-62 cm) 410

16

Lithophile element ratios in this peat segment (e.g., Y/Sc=1.5; Ba/Sc=39 to 43; Nb/Th=0.83 to 411

1.35) are comparable to typical upper continental crust (UCC) compositions (Y/Sc=1.6; 412

Ba/Sc=41; Nb/Th=1.12, Mclennan, 2001), which are close or within the values reported for 413

individual Wicklow granite units in the area (e.g., northern units Leinster batholith, Y/Sc=2.5 to 414

5; Ba/Sc=96 to 160; Nb/Th=0.9 to 7; Fritschele, 2016; Sweetman, 1987). This implies that the 415

local bedrock and soils are the dominant source of elements at this depth, justifying our base-416

line selection. Lead isotope composition of the baseline, 206Pb/204Pb= 18.387±0.012 and 417

207Pb/204Pb= 15.626±0.010 (206Pb/207Pb=1.177±0.011) together with excess Pb concentrations 418

[2] are used to calculate the natural, dust-free Pb isotopy of peat at shallower levels [3]. We 419

note, that similar patterns are obtained by deploying the previously proposed Ti, Zr, or Sc (e.g., 420

Shotyk, 2002) instead of Ta. 421

422

[2] Pbexcess = Pbcon - [(Pb/Ta)sample/(Pb/Ta)baseline] 423

[3] 20xPb/204Pbexcess = [(20xPb/204Pbmeasured * Pbcon) - (20xPb/204Pbbaseline* Pbbaseline)]/Pbexcess 424

425

where the Pbexcess and 20xPb/204Pbexcess are the baseline corrected concentration and isotopic 426

composition of Pb in the polluted samples, Pbcon and 20xPb/204Pbmeasured are the analysed Pb 427

concentration and isotopic compositions, Pb/Tasample is the ratio in the polluted part of the bog, 428

Pb/Tabaseline and 20xPb/204Pbbaseline represent the compositions of the baseline (100-62 cm), and 429

Pbbaseline is the average Pb concentration in the unpolluted part of the bog. For further calcula-430

tions, the subtracted local geogenic contribution was no longer considered. 431

432

4.3 Pollution phase 1 (62-25 cm, pre-1900 AD) 433

Enrichment factors of Cu, Zn, and Pb increase within this segment, which covers the time 434

period before the 20th century (Fig. 5). Importantly, Pb isotope values do not change substan-435

tially from the composition of the baseline (Fig. 4) and are comparable to the published values 436

of the granitic Leinster batholith (Fig. 6a., e.g., Kennan et al., 1987), which is the host rock of 437

local Pb-Ag-Zn mineralisation. The increasing heavy-metal input is thus interpreted to reflect 438

17

the 18th and 19th century Pb-Ag extractions from the high-grade, low-tonnage galena-rich veins 439

exposed in the Wicklow uplands (Rynne, 2015; Schwartz and Critchley, 1997). Smelting was 440

conducted ~15 km NE from LHB (and ~35 NE km from the mining site) at Ballycorus (e.g., 441

Callaghan, 2014; Shepard, 1981), which may have additionally contributed to the overall pol-442

lution load. The Pb isotope signal argues against a Pb contribution from the clearly distinguish-443

able fingerprint of sphalerite-galena mineralisation hosted in the Lower Carboniferous of the 444

Midland basin (e.g., Wilkinson and Eyre, 2005, Fig. 6a.), subjected to small scale extractions 445

during the same time period (Rynne, 2015). The synchronous enrichment onset and develop-446

ment of Zn and Pb (R2=0.96) in this peat section (Fig. 5c.) suggest pollution from the mining 447

activity, especially because of Zn co-occurring in most Pb ore bodies, and due to the shared 448

15% variation of Zn with PC1 (mining and smelting). An important known example of accidental 449

Zn pollution from Pb mining was the giant Broken Hill deposit in south-central Australia, where 450

the relative timing of Pb+Ag vs. Zn extraction is accurately reflected in a mire peat core ca. 451

1,000 km downwind from the mine (Marx et al., 2010). In LHB, the same holds for Cd, which 452

occurs as major impurity within the mined sphalerite and galena ores. 453

Enrichment patterns of Zn depart from those of Pb towards 25 cm depth, pointing to an 454

increasing significance of a source other than mining. The continuing amplification of Zn dep-455

ositions (EF= 12-23), coupled with subtle elevations in Mo, Ni, V (EF= 0.8-1.6, 1.1-1.8, and 456

1.1, respectively) in LHB suggests atmospheric depositions from a non-geogenic source. At-457

mospheric charging with these particular heavy metals has been long associated with the com-458

bustion of liquid and solid fossil fuels (Danihelka et al., 2003; Gallagher et al., 2018; 459

Hjortenkrans, 2008; Huang et al., 1994; Nriagu, 1989; Pacyna and Pacyna, 2001). In is con-460

text, it is well documented that coals were the main fuel source during the entire period of 461

mining activity in the Wicklow uplands (Rynne, 2015), and can thus be considered a potential 462

metal emission source at this site. 463

464

4.4 Pollution phase 2a (25-5 cm, ~1900-2000 AD) 465

Peak pollution period from local mining is documented between 1900 and 1960’s where EF’s 466

of Cu, Ag, Pb, Cd, Sb, and Sn (PC1) reach values of 10, 18, 18, 51, and 14, respectively. Rare 467

18

Earth element depositions are the highest during this period (Appendix B.3), suggestive of an 468

enhanced atmospheric dust fallout from the mineralisation host rock. Silver, Cu, and Sb en-469

richment coincide with early 20th century mining for coinage taking place at different sites 470

around the Wicklow uplands, but also further inland at Silvermines, Co. Tipperary. Stibnite 471

(Sb2S3) and pyrite (FeS2) were extracted at Avoca (Fig. 1). 472

The atmospheric load in redox sensitive elements V, Cr, Mo, and other metals such as 473

Zn and Ni continues to amplify during the second half of the 20th century. While there is the 474

possibility that a minor amount of their atmospheric load relates to the mining, the largest frac-475

tion is attributed to the continuing atmospheric pollution resulting from the combustion of UK 476

sourced coals known to having been extensively used throughout the stages of ore extraction. 477

Due to the similar Pb isotopic compositions of these coals (e.g., 206Pb/207Pb=1.181-478

1.184±0.018; Farmer et al., 1999) compared to the mining-dominated Pb signature (e.g., 479

206Pb/207Pb= 1.177±0.011), it is not possible to discern these two sources from the Pb isotopy 480

of the peat. Thus, in this case, coal pollution can indeed only be inferred from coupled trace 481

element enrichments. 482

A notable change in the geochemistry of the LHB peat (i.e. element ratios, Fig. 5a-d.), 483

specifically, a decrease in Pb and Ag (PC 1) and an increase in Zn, Mo, and Ni (PC3), is 484

evident from ca. 1940 onwards (Fig. 5). This is accompanied by a prominent turn towards a 485

less radiogenic Pb isotopy (Fig. 3), likely reflecting the transition from mining originated Pb to 486

pollution from the hemispheric introduction of leaded gasoline. To disentangle the Pb contri-487

butions from the two sources, we deployed a binary mixing calculation between the following 488

end-members: A, pollutant 1 (206Pb/204Pb =18.30, 207Pb/204Pb =15.62, 206Pb/207Pb=1.174] min-489

ing/industrial signal of the early 20th century), and B: pollutant 2 (206Pb/204Pb= 17.20, 490

207Pb/204Pb =15.54, 206Pb/207Pb=1.109] average European-Australian leaded gasoline). The 491

modelled curves (Fig. 7b. and described in Appendix D) were calculated using different Pb 492

concentrations of the respective end-member pollutants (10-200 µg g-1). Mixing calculations 493

expose the increasing influence of leaded gasoline over the course of 20th century, becoming 494

progressively more accentuated only after the decline of the local mining and smelting activity 495

(post 1940, 18 cm). Quantifying potential contribution of vehicle emissions before 1940s is 496

19

complicated by the masking from concurrent mining activities which lasted until ca. 1957 497

(Rynne, 2015). The least radiogenic 206Pb/207Pb values of 1.138±0.011 recorded in peat from 498

the 1970-80s represent the peak Pb contribution from leaded petrol documented at this site 499

(Fig. 7a.). Because of the large variation in the Pb isotopic composition of ores used for the 500

manufacture of petrol additives (206Pb/207Pb=1.06-1.12, Towsend et al., 1998; Veron et al., 501

1999), there is a significant uncertainty of 6 to 78% in the calculated contributions (Fig. 7b.). 502

Similar findings were made in studies of bogs and freshwater lake sediments from central 503

Scotland (e.g., Farmer et al., 1997). 504

The decrease in the Pb pollution load as documented by LHB during the main period of 505

leaded gasoline, 1950-1980 (Fig. 3), is unusual by international comparison (e.g., see Martinez 506

Cortizas et al., 2002). However, this appears to be an effect caused by previous local mining 507

that masks the gasoline signal from the first half of the 20th century. Only when mining activity 508

subsided and overall Pb EF dropped, did gasoline become a significant contributor. The at-509

mospheric Pb concentrations during the peak Pb gasoline period are, in fact, comparable to 510

those in other Irish archives (Schell et al., 1997). The high load in Ni, Mo, Cr, V and Zn docu-511

mented between 1940 and 1980, support the increasing atmospheric influence of liquid fossil 512

fuels. Due to their versatile nature, i.e., coals, heating oil, peat, gasoline, used for industrial 513

and domestic purposes in Ireland, it is not possible to depict exact fingerprints of fuel groups 514

with the data available. 515

516

4.5. Pollution phase 2b (5-0 cm, 2000-2015 AD) 517

Despite continuing Pb-Zn (Tynagh, Navan, Silvermines, Galmoy, and Lisheen) and Cu-518

Ag-Hg (Ballynoe and Gortdrum) mining from the 1970’s into the 21st century at different sites 519

in Ireland, there is poor correspondence between enrichment of these metals within LHB and 520

the reported tonnage extractions for the post 1980 AD period (see Appendix B.4). This points 521

to stronger atmospheric influence of other (additional) sources at this site today. Notably, this 522

is probably because i) metal extraction sites are situated further away in the Midlands, ii) most 523

ores are extracted underground, and iii) smelting in Ireland has ended, leading to historically 524

lowest atmospheric Zn-Pb pollution from mining (PC1). 525

20

Persisting Zn, Ni, Mo, Cr, and V depositions despite international leaded gasoline aboli-526

tion corroborate that Pb petrol is just one component of the larger pollutant group controlling 527

PC3, emissions from fossil fuels. For example, sub-surface enrichments of Cu and Ni and their 528

ratios including Pb (Pb/Ni= 3-6.7 and Cu/Pb= 0.4-1) are within the experimentally determined 529

values of the aerosols and particulate matter (PM) resulting from combustion of peat (1.8-14.6 530

and 0.6-1, Othman and Latif, 2013). Turf (cut and dried peat) is an important heating fuel in 531

most rural areas of Ireland, and seasonal harvesting for domestic purpose takes place also at 532

the peripheries of LHB. Although Ireland documents a strong historic and recent dependence 533

on different kinds of fossil fuels, their atmospheric influence likely became increasingly more 534

pronounced over the past ca. 60-70 years, i.e., with the introduction of the leaded gasoline 535

(first half of 20th century), and cessation of the mining activity. 536

With the abolition of leaded gasoline in Europe, North America and in 2000 in Ireland, 537

Pb isotopic compositions of LHB peat progressively returned to more radiogenic values (Fig. 538

7a.). However, these ratios (206Pb/204Pb=17.929±0.007; 207Pb/204Pb=15.613±0.007; 539

[206Pb/207Pb=1.148±0.007]) are far from the “baseline” signal, implying either, i) a slow post-540

gasoline amelioration of atmospheric Pb pollution load; ii) recent pollution from a source with 541

partially 206Pb depleted signature; iii) re-suspension of legacy Pb from regional erosion (Clo-542

quet et al., 2006); or iv) a combination of these factors. Shotyk and Krachler (2010) attributed 543

the post-1975 to present Pb isotope variations in the European atmosphere to the growing 544

importance of modern industrial processes and urbanisation relative to decline of leaded petrol 545

use. The compilation of Pb-isotope ratios of potential present-day sources shown along with 546

the LHB values in Figure 6b point towards recent pollution from vehicle exhausts of EU origi-547

nated non-leaded petrol (Erel et al., 1997; Hurst et al., 2002) and atmospheric trace metal 548

release from the use of mineral oil (Chiaradia and Cupelin, 2000). The latter constitutes the 549

major energy source for domestic heating in Co. Dublin. There is also a strong positive coher-550

ence between the increasing amount of private and public vehicles in Ireland (www.cso.ie), 551

especially around Dublin and the rising dominance of PC3 - fossil fuels fingerprint, in the Irish 552

atmosphere, supporting the presence of a growing real pollution signal, rather than remobili-553

sation of previously deposited pollutants. Lead isotope ratios in the subsurface layer of LHB 554

21

are consistent with the estimated average composition of what is suggested to be the signal 555

of the present-day atmospheric Pb pollution (206Pb/207Pb=1.14-1.16, Carignan et al., 2005; 556

Monna et al., 1997), calculated using the fly ash compositions of waste incinerators in central 557

Europe. There are thus a variety of recent pollution sources which can explain the Pb isotope 558

composition of the atmosphere. 559

560

4.6. Timing of Pb pollution and synthesis of Pb isotope information from other rele-561

vant peat archives 562

Due to its location at the east coast of Ireland, LHB records processes that are part of broader 563

regional and hemispheric pollution patterns. In order to facilitate a comparison between the Pb 564

pollution history recorded by LHB and other Irish archives (Coggins et al., 2006; Schell et 565

al.,1997), accumulation rates were calculated according to the method described in detail by 566

Muller et al. (2008). The results are presented in Table 1. In LHB, Pb deposition rates range 567

between 0.45-1.8 µg cm-2 yr-1, with values >1 µg cm-2 yr-1 observed at different stages through-568

out the 20th century (Fig. 8). Peak accumulation rates are detected between 1960’s and 1980’s, 569

in excellent agreement with the Pb deposition rates in a peat archive situated ca 1-2 km away 570

to the west, Kippure bog (Schell et al., 1997). Lead accumulation rates are higher in the LHB 571

peat deposited during the early 20th century mining period, which is likely due to its relative 572

proximity to the mining and smelting sites. Enhanced Pb deposition during the 1980’s can be 573

attributed to the leaded gasoline peak, as it can also be seen from the Pb isotope composition 574

of LHB. In the Midlands (Clara bog), Pb deposition rates were up to 3 times lower than at the 575

eastern seaboard (Fig. 8), yet still higher than at all sites investigated so far along the west 576

coast (Knockroe, Co. Mayo 0.1-0.3 µg cm-2 yr-1, Letterfrack, Co. Galway 0.4-0.5 µg cm-2 yr-1, 577

Coggins et al., 2006; and Bellacorick, Co. Mayo, 0.05-0.4 µg cm-2 yr-1 Schell et al., 1997). It 578

appears that historical mining and smelting have not significantly influenced the archives on 579

the west, although there is also the possibility that the clean Atlantic winds dominating here 580

contribute to an overall dilution of atmospheric pollution. Peak Pb deposition at the west coast 581

occurs slightly earlier during the 1960’s and has been previously interpreted to render the long-582

22

range transport of polluting Pb from the American continent. We suggest that further investi-583

gations, such as the Pb isotopy of these westernmost archives are necessary before exact 584

conclusions can be drawn. A trans-Atlantic Pb gasoline pollution during the 1960’s would an-585

ticipate the isotope signal of the peat to be close to those of Canadian (206Pb/207Pb= 1.15-1.17, 586

Bathurst) and USA (206Pb/207Pb= >1.28, e.g., Mississippi) ores. 587

In general, the timing of Pb isotopic excursions in LHB core coincide with those in bogs 588

from Scotland (e.g., Farmer et al., 2002), Sweden (Bindler et al., 2004), or Spain (Martinez-589

Cortizas et al., 2012). All these archives commonly report a 206Pb/207Pb of 1.16-1.18 in the 590

lower sections (i.e., geogenic influx, and/or mining), and a rapid change towards less radio-591

genic 206Pb/207Pb of 1.13-1.14 at peak Pb-gasoline pollution (1970 to 1980 AD). Because of 592

the different chronology of recent atmospheric pollution in North America (e.g., Bindler et al., 593

2001; Pérez-Rodríguez et al., 2018; Weiss et al., 2002), it is very likely that the east coast of 594

Ireland was, and currently is dominantly influenced by the central- and north-European pollu-595

tion circulation rather than trans-Atlantic sources. 596

597

5. Summary and conclusion 598

We investigated the atmospheric elemental and Pb isotope evolution resulting from local min-599

ing, industrial activities and leaded gasoline pollution in eastern Ireland over the last century 600

from the geochemistry of a bog in the Wicklow Mountains. The elemental cycle associated with 601

mining of local ores (e.g., Pb, Cu, Ag, Sn, Sb) was highly perturbed during the major period of 602

mining and smelting in the Wicklow area (19th to 20th century), showing up to 20-fold, 15-fold, 603

and 50-fold enrichments (Pb, Ag, and Sb, respectively) with respect to the established baseline 604

of the core. Element deposition associated with the combustion of fossil fuels (Zn, V, Ni, Cr 605

and Mo) is detected throughout the polluted section of the core. This pollutant becomes in-606

creasingly more pronounced only after the abolition of the mining activity in the area (ca. 1940-607

recent). The Pb isotopes precisely document the shift towards less radiogenic 206Pb/204Pb val-608

ues in response to the introduction of the Pb petrol additives from ca. 1940 (in this archive) 609

until its complete elimination in 2000 in Ireland. In the most general sense, the observed iso-610

topic shift (206Pb/207Pb=1.138±0.011 in 1970) is consistent with findings in other records of 611

23

atmospheric Pb deposition around Europe (e.g., Sweden, Brännvall et al., 1997; Scotland, 612

Farmer et al., 1997; Spain, Kylander et al., 2005; Switzerland, Weiss et al., 1999). However, 613

unlike at most other sites, the introduction of leaded petrol is not associated with the highest 614

atmospheric Pb pollution load at this site, which was instead caused by the historical Pb-Zn 615

mining and smelting of local ores. Despite a notable decline in the leaded gasoline signal after 616

2000, the Pb isotopic composition has not quite returned to pre-industrial values, implying input 617

from modern pollutant sources. Based on combined trace element and Pb isotope investiga-618

tions, we suggest that coal, peat, and oil combustion, emissions from unleaded petrol, as well 619

as trans-regional industrial pollution (e.g., waste incinerators) potentially reaching Ireland on 620

easterly airstreams are the sources contributing to aerial Pb pollution at this site today. The 621

strong local control of the bog geochemistry demonstrated in this study emphasises the need 622

to combine metal concentration and isotopic investigations for reconstructing historic pollution. 623

Without having investigated the Pb isotopic patterns, the major Pb enrichment peak could have 624

been erroneously attributed to an early Pb gasoline signal. The decline in most heavy metals 625

(esp. Pb, Cu, Sb) in the present day Sphagnum moss is in line with observations made in bogs 626

from elsewhere (e.g., Switzerland, Shotyk et al., 2001), and can be attributed to tougher air 627

pollution prevention policies adopted in Europe over the last three decades. 628

629

Acknowledgements 630

631

The research leading to these results has received funding from the People Programme (Marie 632

Currie Actions) IsoNose (www.isonose.eu) of the European Union’s Sevenths Framework Pro-633

gramme FP7/2007-2013/under REA grant agreement no [608069]. We are thankful for con-634

structive comments from two anonymous reviewers which helped us to improve the clarity and 635

structure of this manuscript significantly. 636

637

24

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920 http://www.cso.ie/en/releasesandpublications/ep/p-eii/eii2016/energy/ 921

http://www.cso.ie/en/statistics/transport/vehicleslicensedforthefirsttime/ 922

Geological Survey of Ireland (GSI): https://www.gsi.ie/Publications+and+Data/ 923

924

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Figure captions 925

926

Figure 1: Map of Ireland showing the geographic distribution of peatlands (brown areas), com-927

piled using data from GSI-Ireland (www.gsi.ie), the sampling site LHB (red dot) along with the 928

historic Pb-Zn mining and smelting sites in the Wicklow mountains (yellow squares) and the 929

historic Pb, Cu, S mines at Avoca (purple square). Bog sites discussed in the studies of Cog-930

gins et al (2006): Letterfrack (LF) and Knockroe (KN), and Schell et al. (1997): Bellacorick (BL), 931

Clara (CL), and Kippure (KP) are indicated as green circles. The main Pb-Zn ore deposits 932

hosted within the Lower Carboniferous of the Midland basin are indicated by blue triangles. 933

Blue arrows represent the wind directions. 934

935

Figure 2 a) and b): Radionuclide (210Pb and 241Am) activity distribution within the LHB peat 936

core with errors at 1σ. Two additional samples investigated at 72 cm and 95 cm depth dis-937

played no 210Pb-241Am activity and are not plotted here. Radioactive equilibrium was reached 938

at 24 cm from the surface. c) Density-, and d) Percent organic matter content along the LHB 939

peat core. e) Age-depth relationship as received from the 210Pb CRS model. The 1 sigma un-940

certainty in 210Pb values including error propagation from calibrations is 10-12% (error bars). 941

A combination of the 241Am maximum and the 210Pb inventory was used for the calculation of 942

the final core chronology as detailed in the text. 943

944

Figure 3: Metal enrichment factors along the LHB peat core (grey circles) displayed relative to 945

the baseline (EF) defined through the lowermost 4 samples (100-62 cm). The chronology (AD) 946

obtained from the 210Pb-241Am activity profiles is indicated in blue. 947

948

Figure 4: Pb isotope ratios (black solid circles) together with EF of Cu, Zn and Pb (open dia-949

monds) along the LHB peat core. Depth ranges distinguished by colours represent changes in 950

either Pb isotopic composition, metal concentrations, or both. Radionuclide derived chronology 951

and contemporaneous historical anthropogenic activities are also indicated. 952

31

953

Figure 5: Bivariate plots of selected metal EF of peat from the LHB core. Samples from the 954

lower part of the core (prior the introduction of the Pb gasoline from 1940) are shown as solid 955

black circles. Peat deposited from the 1940s until present day are shown as white open trian-956

gles. Grey arrows indicate upward direction from the base of the core to the top, i.e., from local 957

background through the mining period, Industrial Revolution (IR), Pb gasoline to present day. 958

959

Figure 6: a) Common Pb (207Pb/204Pb vs. 206Pb/204Pb) plot of LHB peat subsamples and liter-960

ature values of local mineralisation: [1] Wicklow granites, Kennan et al. (1986), and the Mid-961

lands mineralisation [2] Hitzman et al. (1992), and Wilkinson et al. (2005). A sharp change in 962

the Pb isotopic composition is observed from ca. 1940; b) Zoomed out common Pb diagram 963

with pre-polluted peat samples (black solid circles) and all polluted LHB samples (white dia-964

monds) together with the values of the local mineralisation. Also shown are Pb isotope ratios 965

of pre-pollution at other sites (Klaminder et al., 2003; Kylander et al., 2005; Shotyk et al., 2002; 966

2010; Rosman et al., 2000) and for different anthropogenically derived pollutants compiled 967

from Chiaradia and Cupelin, 2000; De Vleeschouwer et al., 2009; Hansmann and Koppel, 968

2000; Monna et al., 1997; Sudgen et al., 1993; Towsend et al., 1998; Veron et al., 1999; 969

Walraven et al., 1997). Error bars for all LHB Pb isotope values are smaller than the symbols. 970

971

Figure 7: a) Chronology-integrated 206Pb/204Pb isotope composition of the upper part of the 972

LHB peat profile together with areas representative of the local mineralisation (Kennan et al., 973

1986) and potential anthropogenic pollution sources during the 19th to 21st century. b) Common 974

Pb isotope binary mixing model using A: average Pb isotope values of local mineralisation 975

(206Pb/204Pb=18.300; 207Pb/204Pb=15.622, Kennan et al., 1986), and B: average compositions 976

representative for aerosols associated with Pb enriched vehicle emissions 977

(206Pb/204Pb=17.200; 207Pb/204Pb=15.543, e.g., Chiaradia and Cupelin 2000; Monna et al., 978

1997; Veron et al., 1999). Lead concentrations range between 10 and 200 µg g-1, correspond-979

ing to values measured along the LHB peat profile. For the calculation steps refer to the text 980

and Appendix D. Error bars are smaller than the symbols. 981

32

982

Figure 8: Lead accumulation rates µg cm-2 yr-1 in the upper part of the LHB (this study). Shown 983

are also accumulation rates calculated in other peat archives from Ireland (Schell et al., 1997). 984